A variety of devices in a wide range of industries include one or more components prepared from a material that undergoes a phase change at a temperature of interest. The phase change alters the physical properties of the component part, which typically alters the way other components in a device are positioned or interact. For example, in response to a phase change in a structural component of a device, related components may expand or contract, inducing a reaction, such as movement of an adjacent component part.
Common examples of such devices are mechanical actuators in which a material, typically a metal alloy, a polymer, or a wax, in the actuator expands or contracts to effect movement of parts in the actuator (e.g., U.S. Pat. Nos. 5,025,627; 5,177,969). Temperature relief valves having a thermal trigger composed of a eutectic material are known (e.g., U.S. Pat. No. 5,495,865). Fire sprinklers and fire extinguishes having a component part that responds to temperature increases are known (e.g., U.S. Pat. Nos. 4,896,728; 4,006,780).
Temperature indicating devices for use in the medical industry and in the food industry are also known, in which a component in the temperature-indicating device is composed of a material that undergoes a phase change at a selected temperature (e.g., U.S. Pat. Nos. 4,289,088; 4,170,956; 5,537,950; 5,988,102; 6,403,131). In such devices, a spring loaded indicator is typically held in place by a small quantity of solid or semi-solid material that melts or deforms at a temperature of interest, thereby allowing the device to respond to a preselected temperature. Exemplary materials are a eutectic metal alloy or an organic compound.
These and similar devices require the use of materials that have sharp melting points, typically in the range of 50 to 100° C., where most foods are cooked and within the range of environment and physiological temperatures.
However, there are few eutectic metal alloys that have a melting temperature in this temperature range. In addition, eutectic metal alloys such as, e.g., lead/cadmium mixtures, have inherent toxicity and may not be suitable for use in the medical or food industries. Low melting eutectic alloys are also costly.
Similarly, there are few organic compounds that have melting points in the temperature range of interest, and of those that do have a suitable melting point often possess a physical or chemical property that renders the material toxic or otherwise undesirable for use in an actuator device.
A further problem with existing metallic and organic compounds is that it is not possible to vary the melting point, while at the same time maintaining abrupt melting behavior (i.e., a sharp melting curve). For example, if the composition of an eutectic alloy is changed slightly, the melting point will either not change, broaden unacceptably, or give multiple melting points. Likewise, if a pure organic compound having a given melting point is mixed with another compound, the melting point of the mixture will invariably be lower and occur over a broader range. Moreover, while melting is a thermodynamic property, the behavior of a mechanical device, such as a food temperature indicator or a fire sprinkler, is also determined by the mechanical properties of the materials used. Many organic compounds that have sharp melting points do not provide the mechanical properties for use in a thermo-mechanical device.
It would be desirable to have a series of organic materials for which the melting properties could be varied while maintaining a sharp melting profile, and which possess the necessary mechanical properties for use in a thermo-mechanical device. Ideal materials (i) have a melting point between about 50 and about 120° C.; (ii) are non toxic to humans; (iii) have a low vapor pressure at the temperatures at which they are used; (iv) are stable at elevated temperatures and humidity; (v) have minimal odor; (vi) have a rapid rate of crystallization; (vii) are low in cost; (viii) are readily produced in high purity; and (ix) demonstrate a sharp melting profile characterized by an abrupt change in viscosity with a temperature change.